The present disclosure relates to continuous cementitious board manufacturing processes and, more particularly, to a system and method for measuring the degree to which cementitious slurry has set at a predetermined point along the manufacturing line during its manufacture.
In many types of cementitious articles, set gypsum (calcium sulfate dihydrate) is often a major constituent. For example, set gypsum is a major component of end products created by use of traditional plasters (e.g., plaster-surfaced internal building walls), and also in faced gypsum board employed in typical drywall construction of interior walls and ceilings of buildings. In addition, set gypsum is the major component of gypsum/cellulose fiber composite boards and products, as described in U.S. Pat. No. 5,320,677, for example. Typically, such gypsum-containing cementitious products are made by preparing a mixture of calcined gypsum (calcium sulfate alpha or beta hemihydrate and/or calcium sulfate anhydrite), water, and other components, as appropriate to form cementitious slurry. The cementitious slurry and desired additives are often blended in a continuous mixer, as described in U.S. Pat. No. 3,359,146, for example.
In a typical cementitious board manufacturing process such as gypsum wallboard, cementitious board is produced by uniformly dispersing calcined gypsum (commonly referred to as “stucco”) in water to form aqueous calcined gypsum slurry. The aqueous calcined gypsum slurry is typically produced in a continuous manner by inserting stucco and water and other additives into a mixer which contains means for agitating the contents to form a uniform gypsum slurry. The slurry is continuously directed toward and through a discharge outlet of the mixer and into a discharge conduit connected to the discharge outlet of the mixer. Aqueous foam can be combined with the aqueous calcined gypsum slurry in the mixer and/or in the discharge conduit. A stream of foamed slurry passes through the discharge conduit from which it is continuously deposited onto a moving web of cover sheet material (i.e., the face sheet) supported by a forming table. The foamed slurry is allowed to spread over the advancing face sheet. A second web of cover sheet material (i.e., the back sheet) is applied to cover the foamed slurry and form a sandwich structure of a continuous wallboard preform. The wallboard preform is subjected to forming, such as at a conventional forming station, to obtain a desired thickness.
The calcined gypsum reacts with the water in the wallboard preform to form a matrix of crystalline hydrated gypsum or calcium sulfate dihydrate and sets as a conveyor moves the wallboard preform down the manufacturing line. The hydration of the calcined gypsum provides for the formation of an interlocking matrix of set gypsum, thereby imparting strength to the gypsum structure in the gypsum-containing product. The product slurry becomes firm as the crystal matrix forms and holds the desired shape.
After the wallboard preform is cut into segments downstream of the forming station at a point along the line where the preform has set sufficiently, the segments are flipped over, dried (e.g., in a kiln) to drive off excess water, and processed to provide the final wallboard product of desired dimensions. The aqueous foam produces air voids in the set gypsum, thereby reducing the density of the finished product relative to a product made using a similar slurry but without foam.
In general, the hydration rate can impact the final strength and production speed of the gypsum-containing product. Furthermore, in the process for making cementitious boards, the setting and drying steps are the most intensive in terms of time and energy. The setting time of the slurry depends on a number of factors, including the age of the calcined gypsum, impurities in the calcined gypsum, surface area, pH, particle size, and the temperature at the time of mixing. Different additives and/or process condition changes can be employed to influence the hydration rate of the slurry to ensure that the cementitious board being produced is suitable for its intended purpose. Accordingly, it is desirable for an operator to determine the rate of set that the cementitious slurry undergoes as it progresses along the line.
Conventionally, an operator at a cutting station, which is located downstream of the forming station at a position where the slurry is expected to have sufficiently set so that the wallboard preform can be cut into segments, uses a “thumb test” to periodically monitor the hydration rate (or set time) of the slurry. The thumb test comprises pushing one's thumb on the back of the board to feel how firm it is. An operator must be in attendance to measure the hardness periodically by pressing the board with a thumb, and the operator decides by experience whether the board is sufficiently set. It can be difficult to compare data from thumb tests taken at different times and/or by different operators.
Also, it is known to monitor the temperature rise of the slurry as it sets. The temperature of the cementitious slurry rises during the setting process, which is an exothermic reaction that generates heat. The temperature of the slurry increases over time, eventually reaching a maximum temperature as the hydration reaction moves toward completion. A temperature rise set (TRS) curve can be plotted which tracks temperature over time so that an operator can determine a hydration percentage for the slurry at various points along the machine line. However, obtaining temperature rise set data on a continuous basis can be difficult, including overcoming difficulties caused by ambient temperature or other influences.
There is a continued need in the art to provide additional solutions to enhance the production of cementitious boards. For example, there is a continued need for techniques for monitoring the set of cementitious slurry during the manufacture of a cementitious board.
It will be appreciated that this background description has been created by the inventor to aid the reader and is not to be taken as an indication that any of the indicated problems were themselves appreciated in the art. While the described principles can, in some aspects and embodiments, alleviate the problems inherent in other systems, it will be appreciated that the scope of the protected innovation is defined by the attached claims and not by the ability of any disclosed feature to solve any specific problem noted herein.